Add --pkgimages=existing.

[maleadt]

Equivalent of #50586; implements #51474.

With --pkgimages=existing, it's possible to disable the (often slow) generation of package images, without losing the ability to use existing ones. That's important now that we're moving more and more packages outside of the system image, e.g., running with --pkgimages=no otherwise takes close to 30s here before the Pkg REPL is usable.

The main motivation for this is PkgEval, where generating package images is not very useful, yet disabling generation of them makes each job (which requires Pkg to drive the test process) take a significantly longer time.

For example, --pkgimages=yes vs no:

❯ JULIA_DEBUG=loading ./julia --project=Example.jl --pkgimages=yes
# no precompilation of REPL.jl
┌ Debug: Loading object cache file /Users/tim/Julia/src/julia/build/dev/usr/share/julia/compiled/v1.11/REPL/u0gqU_XmENM.dylib for REPL [3fa0cd96-eef1-5676-8a61-b3b8758bbffb]
└ @ Base loading.jl:1116

julia> using Example
# short time precompiling + pkgimg generation for Example.jl
┌ Debug: Loading object cache file /Users/tim/.julia/compiled/v1.11/Example/lLvWP_tJaso.dylib for Example [7876af07-990d-54b4-ab0e-23690620f79a]
└ @ Base loading.jl:1116

❯ JULIA_DEBUG=loading ./julia --project=Example.jl --pkgimages=no
┌ Debug: Rejecting cache file /Users/tim/Julia/src/julia/build/dev/usr/share/julia/compiled/v1.11/REPL/u0gqU_XmENM.ji for REPL [3fa0cd96-eef1-5676-8a61-b3b8758bbffb] since the flags are mismatched
│   current session: use_pkgimages = false, debug_level = 1, check_bounds = 0, inline = true, opt_level = 2
│   cache file:      use_pkgimages = true, debug_level = 1, check_bounds = 0, inline = true, opt_level = 2
└ @ Base loading.jl:3289
# long time precompiling REPL.jl
┌ Debug: Loading cache file /Users/tim/.julia/compiled/v1.11/REPL/u0gqU_CWvWI.ji for REPL [3fa0cd96-eef1-5676-8a61-b3b8758bbffb]
└ @ Base loading.jl:1119

julia> using Example
# short time precompiling Example
┌ Debug: Loading cache file /Users/tim/.julia/compiled/v1.11/Example/lLvWP_CWvWI.ji for Example [7876af07-990d-54b4-ab0e-23690620f79a]
└ @ Base loading.jl:1119

With the new --pkgimages=existing:

❯ JULIA_DEBUG=loading ./julia --project=Example.jl --pkgimages=existing
# no precompilation of REPL.jl
┌ Debug: Loading object cache file /Users/tim/Julia/src/julia/build/dev/usr/share/julia/compiled/v1.11/REPL/u0gqU_XmENM.dylib for REPL [3fa0cd96-eef1-5676-8a61-b3b8758bbffb]
└ @ Base loading.jl:1116

julia> using Example
# short time precompiling Example
┌ Debug: Loading cache file /Users/tim/.julia/compiled/v1.11/Example/lLvWP_CWvWI.ji for Example [7876af07-990d-54b4-ab0e-23690620f79a]
└ @ Base loading.jl:1119

[vchuravy]

LGTM, The difference to --compile-modules=existing is that we would still create .ji files?

Would be nice to update docs (and maybe news) as well.

[maleadt]

The difference to --compile-modules=existing is that we would still create .ji files?

Yes, AFAICT creating those doesn't cause a significant overhead, so can be left enabled on PkgEval (i.e., it's just adding some additional granularity).

I'll look into creating docs and updating news once this passes initial review.

[vchuravy]

Might make sense to add a CLI test as well for the flag parsing.